Breast augmentation or reconstructive surgery may employ implants containing silicone. The silicone used in breast prostheses is composed of poly-dimethylsiloxane with varying degrees of polymerization. Dow Corning's implants are approximately 40% polymerized.
Rupture and leakage of the membrane containing the silicone is a known complication of these procedures. The prevalence of complications is not known because patients may be asymptomatic, however, in light of anecdotal reports of a possible link between silicone leakage and systemic autoimmune disease, is important to develop a sensitive noninvasive method to detect leaks.
The leak or rupture may occur anywhere over the surface of an implant and therefore the use of three-dimensional medical imaging techniques is desirable. Such imaging would, in theory, allow careful scrutiny of the entire surface of the implant and the detection of even small pockets of migrating silicone near that surface.
NMR imaging ("MRI") is one technique capable of the necessary three dimensional imaging. A uniform magnetic field B.sub.0 is applied to an imaged object along the z-axis of a Cartesian coordinate system, the origin of which is within the imaged object. The effect of the magnetic field B.sub.0 is to align the object's nuclear spins along the z-axis. In response to radio frequency (RF) pulses of the proper frequency oriented within the x-y plane, the nuclei resonate at their Larmor frequencies according to the following equation: EQU .omega.=.gamma.B.sub.0 ( 1)
where .omega. is the Larmor frequency, and .gamma. is the gyromagnetic ratio which is a property of the particular nucleus. Water, because of its relevance abundance in biological tissue and the properties of its proton nuclei, is of principle concern in most imaging. The value of the gyromagnetic ratio .gamma. for protons in water is 4.26 kHz/Gauss and therefore in a 1.5 Tesla polarizing magnetic field B.sub.0, the resonant or Larmor frequency of water protons is approximately 63.9 MHz. The other primary constituent of biological tissue is fat. Larmor frequency of protons in fat is approximately 203 Hertz higher than that of the protons in water in a 1.5 Tesla polarizing magnetic field B.sub.0.
In the well known slice selective MRI sequence, a z-axis magnetic field gradient, G.sub.z is applied at the time of an RF pulse so that only the nuclei in a slice through an object in the x-y plane are excited into resonance. The coherence between the nuclei decays as characterized by two relaxation times T.sub.1 and T.sub.2. After excitation of the nuclei, magnetic field gradients are applied along the x and y axes and an NMR signal is acquired. The gradient field along the x-axis, G.sub.x, causes the nuclei to precess at different resonant frequencies depending on their position along the x-axis; that is, G.sub.x spatially encodes the precessing nuclei by frequency. Similarly, the y-axis gradient, G.sub.y, encodes y position into the change of magnetization or NMR signal phase as a function of G.sub.y gradient amplitude. This process is typically referred to as phase encoding.
From this data set, an image may be derived according to well known reconstruction techniques. The image comprises an array of complex pixel values having magnitude and phase. Typically the magnitudes of the pixels are mapped to a gray scale to form the visual image.
In a 1.5 Telsa B.sub.0 field the Larmor frequency of the silicone protons is approximately 102 Hertz higher than the protons of fat and 305 Hertz higher than the protons of water. The difference between the Larmor frequencies of different isotopes or species of the same nucleus, viz., protons, is termed chemical shift, reflecting the different atomic environments of the species.
As noted above, the silicone used in such breast implants is composed of poly-dimethylsiloxane with varying degrees of polymerization. The primary NMR signal is from magnetically equivalent protons on the methyl groups which rapidly rotate about the Si-C bond axis. The single resonance has fairly long T.sub.1 and T.sub.2 relaxation times. Other protons in the silicone gel are present in very low concentrations (e.g., residual D4 monomers) or have very short T.sub.2 relaxation times (e.g., cross-links) and are not detectable by MR imaging.
Critical to imaging a breast prosthesis is the ability to isolate the silicone signal from the water and fat signals comprising the majority of the breast tissue. In theory, because the silicone protons have a discrete and separate resonance from fat or water protons, the signal from the silicone should be capable of isolation from that of fat and water. Nevertheless, the small difference between the frequency of resonance of the fat and silicone protons at even high field strengths of 1.5 Tesla restricts the use of selective excitation techniques, or saturation of the silicone resonance, to cases of extremely good B.sub.0 field homogeneity.